Process of removing and recovering sulphur dioxide



Dec. 24, 1940.

H. F. JOHNSTONE PROCESS oF REMOVING AND RECOVERING sULPHUR DIOXIDE Filed Aug. 25, 1958 Patented Dec. 24, 1940'- PATENT OFFICE PROCESS F BMOVING AND BECOVERING SULPIIUR DIOXIDE Henry F. Johnstone, Urbana, lill.,- assigner, by. mesne assignments, to Commonwealth Edison Company, Chicago, lll., a corporation of Illinois Application August z5. 193s. serial No. 226.763

6Claims.

This invention relates to the removal and recovery of sulphur dioxide from dilute waste gases such as boiler furnace gases, smelter gases, and

- those from the chemical process industries. This 5. invention constitutes an improvement in the Process for removing and recovering sulphur dioxide from waste gases, disclosed in my United States Patent No. 2,082,006.

AAn object of my invention is to provide an 10 emcient solvent for absorbing sulphur dioxide which will eliminate the resistance of the socalled liquid illm known to exist when sulphur dioxide is absorbed by water.

Another object is to provide a means for regenerating the solvent by removing the dissolved sulphur dioxide in admixture with water vapor, thereby rendering the solvent in a condition suitable for absorbing more sulphur dioxide and simultaneously producing concentrated sulphur dioxide suitable for commercial uses.

A secondary object of my invention is to provide a method for greatly increasing the acidity of a solution by increasing its temperature and of greatly decreasing the acidity on cooling thereof. Such a reversible change has many important uses in chemical processes.

Other objects of my invention will, in part. be obvious and, in part, appear hereinafter.

For a more complete understanding of the nature and scope of my invention, reference may be had to the following detailed description takenI in connection with the accompanying drawing in which the single figure is a flow sheet illustrating oneembodiment of my invention.

In the cyclic process of recovering sulphur dioxide from waste gases by absorbing the gas in an aqueous solution containing a soluble sulphite and blsulphte and releasing the said gas by heating the solution, an excessive quantity of steam must be used for regeneration when treating such dilute gases. This is due to the low temperataure coeillcient of the vapor pressure of sulphur dioxide in equilibrium with the solution. With alkali, or ammonium sulphite-bisulphite solutions the percentage increase in vapor pressure of the sulphur dioxide over any increase in temperature is roughly equal to the percentage increase of the vapor pressure of water over the same range. The maximum ratio of sulphur dioxide to water vapor in the vapors leaving the regenerator, therefore, is approximately equal to the ratio of sulphur dioxide to water vapor in the waste gases. Another disadvantage of the simple solutions is the limitation of the extent of 65 regeneration caused by the low vapor pressure of sulphur dioxide. Furthermore, since some nite vapor pressure of sulphur dioxide must exist in equilibrium with the regenerated solution even after cooling, it is usually impossible to remove completely the sulphur dioxide from the waste 5 gases.

Modifications of the simple sulphite-bisulphite solutions to overcome these diiliculties have been used. For example, using the salt of an organic acid, the acid of which increases in ionization as 1.0 the temperature is raised, provides for greater capacity of the solution and a smaller steam requirement for regeneration. Such solvents for sulphur dioxide are described in my above-mentioned United States Patent 2,082,006. An addl- 15 tional advantage is secured when the organic acid is substantially insoluble at low temperatures and increases in solubility with increases in temperature.

Another method by which the above-mentioned 20 dimculties may be overcome, comprises regenerating the sulphite-bisulphite solutions by a chemical means such as by treatment with a metallic oxide -to precipitate a sulphite which can be decomposed into the said oxide and sulphur 25 dioxide by calcinationV at a relatively low temperature. Such a process is described inthe copending application of Johnstone and Singh, Serial No. 132,692, illed March 24, 1937.

When an organic acid is used as the auxiliary 3o acid, some volatilization of the acid with steam usually occurs. This necessitates additional steps in the process to secure pure sulphur dioxide. Precautionary measures must also be taken when the insoluble acid separates into a second liquid 35 phase to avoid loss by volatilization into the waste gas in the scrubber. Furthermore, many of the acids that have the desired properties are too expensive for use on a large scale. Also,

a specic acid is most applicable to only one con- 40 oentration of sulphur dioxide in the Waste gas.

In the present invention these diiilculties are eliminated by providing for the use of a partially soluble solid inorganic acid as the auxiliary acid whose ionization increases with increase in temperature. Such an acid is boric acid, of which the chemical formula is HaBOa. Boric acid is relatively abundant, cheap, and easily-supplied. Consequently, its use in large quantities is economical. It also has many properties which 50 make it a desirable acid to use as an aid in the regeneration of sulphite-bisulphite' solutions.

It is relatively insoluble in, water, or sulphitebisulphite solutions at normal temperatures, he., 15 to 30 C, lts solubility increases four fold 55 between 30 C. and 100 C. and may be increased even more by raising the temperature above the normal boiling point by heating under pressure. While slightly volatile from concentrated solutions above 60 C., its volatility is negligible lilromits solutions below 50 C. Borlc acid itself is an extremely weak acid, but its great increase in solubility causes it to liberate sulphur dioxide from a hot sulphite-bisulphite solution which has substantially no vapor pressure of sulphur dioxide at 50 C.

It is obvious that the boric acid may be introduced as one of its salts, as for example the sodium salt Na2B4O7.10I-I20, yor borax, as it is commonly called, or as the metaborate, NaBOz.2HnO. In an acid solution of borax, borlc acid will .be formed therefrom. o

Due to its small ionization, boric acid is extremely weak. Its displacement of sulphur dioxide from sulphite-bisulphite solutions, therefore, is not complete even when the molar concentration of the acid is greater than that of sulphur dioxide at the high temperatures. The acid, however, has the property of increasing in strength in the presence of polyhydroxy organic compounds, such as mannitol, sorbitol, dulcitol, glycerol, and the sugars, glucose, levulose, and invert sugar. By the addition of the correct amount of these compounds to the boric acidsulphite-bisulphite solution, more complete displacement of the sulphur dioxide at the high temperatures may be accomplished without materially decreasing the solubility of sulphur dioxide in the solution at the low temperature. The ratio of the polyhydroxy organic compound to the boric acid is preferably adjusted to give an acid of a strength which corresponds to the maximum capacity of the solution for absorbing sulphur dioxide. Too large a ratio renders the boric acid too strong and reduces the capacity of the solution.

EXAMPLES Test No. 1

A solution containing 1.33 moles of sodium and 0.98 mole of sulphur dioxide per liter was boiled under a reflux condenser for ten minutes. No sulphur dioxide was released and the composition remained constant. The same solution Was boiled for ten minutes with 40 g. boric acid per 100 cc. solution.' This quantity of acid was completely soluble at the boiling point. Evolution of sulphur dioxide commenced when the temperature of the solution reached 90 C. After the boiling, the concentration of sulphur dioxide inthe solution was 0.45 mole per liter. On cooling to 30 boric acid separated out as white crystals. The supernatant liquid was heated to 50 C. and was saturated at this temperature with a gas containing approximately 0.5% sulphur dioxide. No crystals of boric acid separated out during the absorption. 'I'he concentration of sulphur dioxide after absorption was 0.70 mole per liter. The solution was mixed with the boric acid crystals and reheated to boiling for 10 minutes. The sulphur dioxide concentration was reduced to 0.38 mole per liter.

The molar ratio oi' alkali metal or alkali metal hydroxide to boric acid or boron in the above test solution during regeneration is- That is, during sulphur dioxide evolution -the lmolar ratio of alkali metal to boron inthe solution is 0.205 to1.0, and, during absorption the ratio of alkali metal to boron is greater than 0.205:1.0 due to crystallization of boric acid on cooling.

Test No. 2

A similar test was made starting with the same sodium sulphite-bisulphite solution, using 20 g. of boric acid per 100 cc. of the solution. After boiling for 10 min. the sulphur dioxide concentration was 0.76 mole per liter. After saturation with the dilute gas at 50, it was 0.98 mole per liter. After boiling the second time for 10 minutes the concentration was again 0.79 mole per liter.

Test No. 3

A similar test. was made starting with the same sodium sulphite-bisulphite solution, using 20 g. of boric acid and 30 g. of mannitolper 100 cc. of the solution. After boiling for l minutes, the sulphur dioxide concentration of the solution was 0.36 mole per liter. After saturation at 50 with dilute gas, it was 0.63 mole per liter. After boiling the second time, the concentration was reduced to 0.29 mole per liter.

These tests show (l) that the addition of borlc acid to a sulphite-bisulphite solution increases the capacity of the solution as a solvent for absorbing sulphur dioxide in a cyclic process comprising absorption at a low temperature and regeneration at a higher temperature, (2) that the ratio of sulphur dioxide to water vapor is/increased by the presence of the boric acid when the vapors leave the regenerator in equilibrium with the influent hot solution, (3) that precipi-` tation of the boric acid inthe scrubber can be prevented by cooling the solution to a. temperature below that of absorption, (4) that the ca-r pacity of the solution for absorbing sulphur dioxide from a given gas increases-With increase in concentration of boric acidin the hot solution, and (5) that the action of a low concentration of boric acid may be supplemented by mannitol to increase the capacity of the solution for the cyclic process.

Application of the invention to the cyclic process of removing and recovering sulphur dioxide is illustrated by the ow sheet shown in the drawing. The gases enter the scrubber i and come in contact with the absorbing solution preferably consisting of an alkali sulphite, sulphiteblsulphite, or sulphite-hydroxide solution. While the scrubbing device shown contains packing to increase the surface of contact between the gas and liquid, any other type of scrubber can be used such as a'spray or a wet cyclone device. Either counter-current or co-current ow may be used in this process, and it is possible and often desirable to maintain a solution throughout the scrubber which has substantially no equilibrium vapor pressure of sulphur dioxide.

When absorbing sulphur dioxide from dustladen gases, it is desirable to separate the suspended dust particles collected from the gas by clarification or by filtration before regeneration of the solution. The solution leaving the scrubber then enters the clarifier 2. The clear overflow passes through the heat exchanger 3 to the regenerator 4. The thickened underflow from the clariiier 2 may be discarded. Preferably it is sent as a side stream to a sulphate removal process since some oxidation of the sulphite to sulphate reduced to" equivalent amount.

usually occurs in the cyclic operation. Furthermore. any sulphuric acid vapor in the waste gases will be absorbed by the solution, and to avoid decreasing the capacity of the solution the soluble sulphate must be removed continuously in an This may be accomplished by the process described in my above-mentioned United States Patent No. 2,082,006, or by a modification thereof. Presence 0I the inert dust in the portion of the solution to be desulphated will not be a disadvantage in this process of desulphating.

Leaving the regenerator 4, the hot solution containing boric acid, or other suitable acid having the characteristics as set forth above. passes to the crystallizer 5 where it is cooled by heat interchange with the colder rich solution. It is further cooled by heat exchange with cold water in the cooler 1. The crystals and mother liquor then pass to the clariiier 6 where they are separated. The crystals in the form of a thick slurry are conveyed by means of the screw device 8 back to the regenerator 4. The clear .regenerated solution is sent back to the scrubber I, from the clarifier 6.

The regenerator may be heated by a steam coil I0 or by direct steam. The vapors leaving the regenerator consist essentially of sulphur dioxide and water vapor. In case the auxiliary acid is volatile with steam, it is desirable to condense the acid, and a portion or all of the water vapor, and return the condensate to the regenerator. This can be accomplished in a condenser 9.

It is understood that boric acid and its compounds with polyhydroxy organic substances serve as examples ofother acids that may be used. Because of its many favorable properties, this auxiliary acid is a desirable one. Others, however, may be used and the invention should be understood to consist of the use of any auxiliary acid which can be substantially completely separated as a solid from the absorbing solution by cooling. A theoretical calculation shows that the auxiliary acids which increase the capacity of a sulphite-bisulphite solution for absorbing sulphur dioxide from dilute gases are characterized by having the product of the ionization constant and the solubility in moles per liter at 25 lie between 10n3 and 1010. It is noted that use of a sulphite-bisulphite solution saturated with such acids gives greater capacity for the cyclic process than use of the unmodified solution whether or not the acid increases in solubility with rise in temperature. If the latter is an additional property of the acid, the capacity will be further increased, and the steam requirements for regeneration will be diminished.

Other acids which have properties similar to those mentioned above, i. e., highly insoluble at absorption temperatures-and increasingly soluble at regeneration temperatures, increased ionization at regeneration temperatures, and precipitating as a solid from the cooled regenerated solution, are benzoic and succinic, both organic. It is understood also that application of the principles outlined above to the removal and recovery of other acid gases canbe made. Thus, the properties of boric acid make it a Vuseful auxiliary agent in releasing carbon dioxide from hot carbonate-bicarbonate solutions. Since the acid is reprecipitated at low temperatures, its presence increases the capacity of these ,solutions for absorbing' carbon dioxide in a cyclic process.

I intend that my invention be limited in scope only as set forth in the appended claims.'

I claim as my invention:

1. The cyclic process of removing and recovering sulphur dioxide from waste gases which comprises contacting said gases with a cooled aqueous solution of a sulphite and a polyhydrox'y organic compound capable of increasing the strength of boric acid when in solution therewith, heating the enriched solution to regeneration temperature with boric acid guiiicient to substantially saturate said solution. thereby releasing the absorbed sulphur dioxide, cooling the solution to precipitate boric acid, separating the precipitated boric acid, returning the solution to contact further with the gases and returning the precipitated boric acid to contact with the enriched solution in a cyclic manner, suicient boric acid being precipitated by said cooling step to prevent further precipitation thereof during further contact o! the solution with the gases.

2. 'I'he cyclic process of removing and recovering sulphur dioxide, from waste gases which comprises contacting said gases with a cooled aqueous solution of a sulphite and mannitol,

- heating the enriched solution with boric acid to increase the concentration 'thereof over its concentration in said cooled solution, thereby releasing the absorbed sulphur dioxide, cooling the solution, separating the precipitated boric acid, returning the solution to contact further with the gases and returning the precipitated boric acid to contact with the enriched solution in a cyclic manner.

3. The cyclic process of removing and recovering sulphur dioxide from waste gases which comprises contacting said gases with a cooled aqueous solution of a. sulphite and sorbitol heating the enriched solution with boric acid to increase the concentration thereof over its concentration in said cooled solution, thereby releasing the absorbed sulphur dioxide, cooling the solution, separating the precipitated boric acid, returning the solution to contact further with the gases and returning the precipitated boric acid to contact with the enriched solution in a cyclic manner.

4. The cyclic process of removing and recovering sulphur dioxide from waste gases which comprises contacting said gases with a cooled aque'- ous solution of a sulphite and glucose, heating the enriched solution with boric acid to increase the concentration thereof over its concentration in said cooled solution, thereby releasing the absorbed sulphur dioxide, cooling the solution, separating the precipitated boric acid, returning the solution to contact further with the gases and returning the precipitated boric acid to contact with the enriched solution in a cyclic manner.

5. In the cyclic process of removing and recovering sulphur dioxide from gases containing the same which comprises absorbing the sulphur dioxide in a cooled absorbing solution and thereafter heating said absorbing solution to regeneration temperature to evolve the absorbed sulphur dioxide, lowering the sulphur dioxide capacity of the heated absorbing solution by introducing into said solution boric acid suillcient to substantially saturate said solution and a polyhydroxy organic compound which increases the acidity of the boric acid at evolution temperation borlc acid and a polyhydr'oxy organic compound capable of increasing the strength of boric acid when in solution therewith selected -from the 'group consisting of mannitol, sorbitol. dulcitol. glycerol, glucose, levulose and invert sugar. 5

HENRY Il".l JOHNSTONE. 

